Polyvinyl chloride (PVC) has been widely used as the covering material of electrical cable because of its superior mechanical strength, extrusion processability, flexibility, and coloring property. However, with recent concern for the global environment, halogen-free resin material has come to be used for covering electrical cables in place of PVC, because PVC discharges a harmful halogen gas on combustion.
Halogen-free resin compositions containing polymers are known in the art for use as electrical cable covering, while having the merit of no generation of a poisonous gas on combustion. Generally, those compositions are based on a polar polymer resin, typically ethylene vinyl acetate (EVA) copolymer, and a hydrated mineral filler. Aluminum trihydrate (ATH) is the most commonly used as a mineral filler and requires high levels, usually about 200-350 phr, to render the required fire performance. To achieve proper dispersion of such high filler loading, compatabilizers are needed, e.g. organosilane surface treatment of the filler or functionalized (e.g. maleic anhydride) polymers.
Metal hydroxides, such as magnesium hydroxide, can also be a substitute for ATH, but a large quantity of a metal hydroxide is also required. That causes problems in reduction of mechanical strength, such as wear resistance, tensile strength and the like. In order to prevent the deterioration of mechanical strength, it may be considered that amounts of a polypropylene having a comparatively high hardness and a high density polyethylene are increased, but the flexibility of the covered electrical wire is reduced thereby and the processability becomes poor.
Examples of halogen free compositions are disclosed in U.S. Pat. Nos. 4,948,669; 4,430,468; 4,434,258; 4,673,620; 4,701,359; and 6,475,628; and U.S. Patent Application Publication No. 2005/0137306.
Therefore, there remains a need for halogen-free compositions for use as a covering material for electrical cables with excellent flame resistance and mechanical properties.